Adrenergic control of transmural coronary blood flow

Tachycardia and an increase in myocardial metabolism result from the sympathetic activation that occurs during baroreceptor reflexes, emotion, and exercise. Paradoxically, a concomitant adrenergic alpha-receptor-mediated coronary vasoconstriction competes with the local metabolic coronary vasodilation that occurs during these conditions, and thereby limits metabolic hyperemia. Measurements of transmural blood flow in alpha-receptor blocked and alpha-receptor intact regions of the left ventricle during exercise demonstrate that adrenergic vasoconstriction helps maintain blood flow to the vulnerable subendocardium during tachycardia. This may be the explanation as to why paradoxical adrenergic coronary vasoconstriction has evolved. During controlled conditions of constant coronary flow, an anti-transmural steal effect due to adrenergic vasoconstriction in the subepicardium can be demonstrated during ischemic conditions. These observations demonstrate unexpected beneficial effects of adrenergic coronary vasoconstriction during tachycardia and cardiovascular stress.